Download Expert Review The Cover Test

Expert Review The Cover Test
Rongxuan Lim* and Peggy Frith#
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The Journal of Clinical Examination (2009) 9: 9-16
Abstract The cover test is indispensable when assessing a patient with a suspected squint. This article reviews
how the cover test is performed and also the limitations of the cover test. Further, information is provided on how
to interpret the clinical signs derived from the cover test. Indeed, much about the squint nature, fixation pattern and
fusion strength of the patient can be learnt from the cover test alone. Word Count: 1701 words (excluding
abstract and references)
Key words: eye examination, cover test, ocular misalignment, strabismus, squint
Address for correspondence: [email protected]
Author affiliations: * Foundation Year 1 Doctor, John Radcliffe Hospital, Oxford. #Consultant Ophthalmologist,
Oxford Eye Hospital, UK.
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Introduction
The cover test is a component of the full
assessment (see Table 1) of a patient with
suspected ocular misalignment. Crucially it enables
the examiner to confirm the presence, direction and
magnitude of the ocular misalignment. This helps
with formulating a management plan for the clinical
problem. This article focuses on how the cover test
is performed and tips for a successful examination.
Literature Search
Major Ophthalmology and Orthoptics textbooks
were consulted and a Pubmed search for relevant
articles was conducted.
Terminology
The visual axis or line of vision is an imaginary line
from the fovea to the nodal point of the eye and to
the fixation point (see Figure 1).
The normal
situation for most people is orthotropia, when using
both eyes a subject’s visual axes intersect at the
point of fixation, O.
In orthophoria, this perfect alignment is still
preserved when binocular vision is interrupted by
covering one eye. Even among “normal” subjects
with no ophthalmic complaints, orthophoria is not
common [1] as most people have a slight degree of
latent malalignment.
Ocular misalignment is also known as strabismus or
squint. However, patients may sometimes mean
something different when complaining of a “squint”,
so it is good practice to always clarify such terms
with patients.
Figure 1 Visual axis and optical axis. O: fixation
point, F: fovea, N: nodal point, AB: optical axis,
OF: visual axis. Modified from [1].
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Heterotropia, also known as manifest or obvious
strabismus/squint, is a misalignment that is present
under binocular viewing conditions - using both
eyes to fixate. Heterophoria or latent
strabismus/squint is misalignment that is only
apparent when fusion of the two eyes is prevented
by covering one eye. Intermittent heterotropia
presents when control of a latent squint is lost
sometimes and the squint becomes present even
when using both eyes. Figure 2 shows the
terminology used to refer to ocular misalignments.
Comitant/concomitant strabismus refers to a squint
that does not vary in magnitude with direction of
gaze nor the fixating eye, usually due to a squint in
childhood. In incomittant/noncomittant strabismus,
the magnitude of deviation varies with direction of
gaze or fixating eye. Most incomittant strabismus
are acquired by paralysis or restriction, making
neurological or orbital disease more likely [2].
Prerequisities for The Cover Test
There are a number of factors relating to the patient
and the equipment which must be in place for the
cover test to be performed properly. These are
outlined below.
Patient Factors
Accurate fixation and co-operation are essential.
The cover test presupposes accurate foveal fixation
and will not be successful if the patient is unable to
reliably fixate due to poor visual acuity (<6/60) or
gross eccentric fixation.
Reasonable attention and co-operation of the
patient is required. When examining young children,
it is essential that the examiner takes effort to form
the necessary rapport with the child [2]. Examining a
squinting child is a highly skilled task, best learned
from an experienced orthoptist.
Equipment Factors
Three factors are important here: the type of
occluder, the type of fixation target and the type of
prisms.
Figure 2 Positions of manifest squints. (a)
Orthotropia (b)Esotropia (c)Exotropia (d)Right
hypertropia
(e)Left
hypotropia
(f)Left
incyclotropia (e)Left excyclotropia. Modified
from [1].
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Several different forms of opaque occluders are
commercially available (see Figure 3). A Spielmann
occluder is translucent and the patient is only able
to see diffuse light without contours though the
occluder. It is sufficient to suspend fusion and
allows the examiner to observe any eye movement
behind the occluder. However, as it is more
expensive than opaque occluders, it is not
commonly used in clinical practice.
Figure 4 Fixation targets for preliterate children.
(Courtesy of the Orthoptic Department, Oxford
Eye Hospital)
Figure 3 Occluders. Left: Two opaque
occluders. Right: A Spielmann occluder.
(Courtesy of the Orthoptic Department, Oxford
Eye Hospital)
Letter charts can be used for an adult. For distance
viewing, a Snellen chart placed at 6m is usually
employed. If the patient’s visual acuity permits, a
6/9 visual acuity symbol should be used as the
fixation target [1]. This produces a precise fixation
point; if a large letter is used, the distance between
different parts of the letter could be substantial and
the patient’s fixation point can vary throughout the
test, giving rise to measurement errors. For near
viewing, the patient is asked to hold a reading card
at a comfortable reading distance (usually 33-40cm)
and to fixate on an alphabet on the card. When
examining preliterate children, a picture or small toy
can be used as both distance and near fixation
targets (see Figure 4).
Although a penlight is sometimes used to assess
the symmetry of corneal reflections as a preliminary
screening test for strabismus (see section:
“Performing the examination”), it should not be used
as a fixation target as it does not control the amount
of accommodation during testing [1].
The prisms used may be loose or in the form of
prism-bars with graded strength of prisms, both
vertical and horizontal (see Figure 5).
Figure 5 Prisms. Top: Loose prism set Bottom:
Prism bars, vertical and horizontal (Courtesy of
the Orthoptic Department, Oxford Eye Hospital)
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Preparation
Start by washing your hands or using alcohol gel.
Introduce yourself to the patient and confirm their
identity, obtain consent to examine them and collect
the necessary equipment.
The patient and the examiner should sit facing each
other at an arm’s length distance (see Figure 6).
Figure 6 Positions of the examiner and the
patient when performing the cover test. The
examiner (right) and the patient (left) are sitting
facing each other at an arm’s length distance.
The examiner places an opaque occluder in
front of the patient’s left eye. With her other
hand, the examiner holds up a near fixation
target for the patient to fixate.
Performing the examination
The full cover test consists of three parts (although
some examiners advocate starting by using the
corneal reflections to gain some preliminary idea
about the squint, both its direction and size – see
Table 2 and Figure 7):
1. Cover test
2. Alternate cover test
3. Prism cover test
Figure 7 Hirschberg test to determine the
direction and estimate the angle of squint - left
esotropia. (a) Orthotropia: bilateral light reflex
centered on pupils (b) 15 degrees esotropia:
light reflex at the edge of left pupil (c) 30
degrees esotropia: light reflex in between left
pupil margin and limbus (d) 45 degrees
esotropia: light reflex at limbus. Modified from [1,
4].
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1) Cover test
The patient is instructed to fixate on the target at all
times. An occluder is then placed in front of one eye
and the non-covered eye is observed (Figure 8).
This is repeated with the second eye. If either noncovered eye shifts to take up fixation, a manifest
squint is present. The direction of the shift will
determine if an exo-, eso-, hypo- or hyper- tropia is
present - if the eye moves in, an exotropia of that
eye is present.
The occluder is then removed, restoring
binocularity. At this point, if the recently uncovered
eye immediately shifts to take up fixation, there is
strong dominance of this eye and amblyopia should
be suspected in fellow eye. If the test is negative on
occluding the first eye, the procedure is repeated
with the fellow eye. Should the test be negative with
both eyes, the patient does not have a manifest
squint and the alternate cover test is performed to
look for a latent squint.
2) Alternate cover test
This test demonstrates the sum of manifest and
latent squints by breaking the fusion of two eyes
looking together. If the cover test determines no
manifest squint, the alternate cover test
demonstrates any latent squint.
The patient is again instructed to fixate on the target
at all times. An occluder is placed in front of one eye
for 2 s, and then switched over to the other eye for 2
s, and then back again. This alternate occlusion is
repeated for several cycles and the examiner
observes any movement of the just uncovered eye.
The principle behind the alternate test is as followsIf a latent squint is present, the just covered eye will
drift away from its usual visual axis (Figure 9). The
examiner does not usually see this if an opaque
occluder is used. When the occluder is removed,
that eye demonstrates a re-fixation shift that the
examiner can detect (Figure 9). The speed and
smoothness of this re-fixation shift indicates the
strength of fusion [3].
Figure 8 Cover test. (a) Patient has exotropia on
inspection, with left eye fixating. (b) Left eye is
covered with occluder. Right eye shifts nasally
to take up fixation. (c) Occluder is removed and
right eye continues to fixate for some time. (d)
Occluder removed and left eye takes up fixation
immediately indicating strong dominance of left
eye. Amblyopia of the right eye should be
suspected.
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3) Prism cover test
Prisms are used to measure the squint
demonstrated in the alternate cover test. First, the
squint magnitude is estimated from the alternate
cover test and a prism with power underestimating
this magnitude is selected. This prism is placed in
front of one eye, with the prism apex pointing in the
same direction as the deviation (eg. apex pointing
nasally for an eso- deviation). Simultaneously, an
occluder is placed in front of the fellow eye.
With the prism in place, do the alternate cover test
again. This should decrease the amplitude of refixation eye movement observed compared to when
no prism was used. The prism strength is increased
until no re-fixation shift is observed. This prism
strength is equivalent to the size of the squint.
Usually, the prism power is further increased such
that the re-fixation movement direction is reversed.
Following which, the prism power is then decreased
again to the point where no re-fixation shift is
observed to confirm the earlier measurement.
Completing the Examination
For a comprehensive examination, each step of the
cover test is performed at near fixation (33-40cm)
and distance fixation (6m), with and without
refractive correction, in the primary position of gaze
(patient look straight forward). Comparison of these
four figures will indicate the contribution of
accommodation to the patient’s strabismus [1] as is
especially important in children with esotropia.
Figure 9 Alternate cover test. (a) Patient is
orthotropic on inspection. (b) When occluding
the left eye, it shifts nasally behind the cover,
demonstrating esophoria. (c) On removing the
cover, left eye shifts out to re-fixate. The speed
of re-fixation indicates the strength of fusion.
To fully suspend fusion, the switching of the
occluder between eyes should be swift such that the
patient is only allowed to have one eye viewing the
target at any one time. Also, it is crucial that
sufficient time is given to the patient to “find and
fixate” on the target each time the occluder is
shifted (2s is just a rough guide).
In patients with strong compensatory innervation
that keeps their eyes properly aligned, the alternate
cover test has to be done for some time before
maximal squint is apparent [1].
To determine comitance/incomitance, the test is
performed in the nine diagnostic positions of gaze
with each eye fixating in turn. This is described in
more detail elsewhere [3]. If weakness of an obliqueacting muscle is suspected, the cover test should
also be performed in left and right head tilt
positions.
Finish your examination by informing the patient
that your examination is complete, thanking them
for their cooperation and ensuring they have
possession of any glasses they may have been
wearing before the examination. Finally ensure
they are comfortable and wash your hands.
Limitations of The Cover Test
The cover test relies on the patient maintaining
accurate fixation and the observer being skilled. In
situations where accurate fixation is not possible, as
for babies, the corneal reflection test (Table 2) may
be used to estimate the size of any manifest squint.
Small amplitude squints may not be picked up by
the examiner. Ludvigh [5] showed that under optimal
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conditions (co-operative patient, experienced
examiner and good lighting), eye movement
corresponding to 2 diopters in prism strength was
the smallest deviation that can be detected.
Cyclodeviations cannot be measured with the cover
test and other methods of detecting ocular
misalignment will need to be used [3].
References
[1] von Noorden GK, Campos ECC. Binocular vision
and ocular motility. 6th ed. Missouri: Mosby; 2002
[2] American Academy of Ophthalmology. Basic and
Clinical Science Course Section 6: Paediatric
Ophthalmology and Strabismus. 2007-2008 ed. San
Francisco: American Academy of Ophthalmology;
2007
[3] Kanski JJ. Clinical Ophthalmology: A systemic
approach. 6th ed. Edinburgh: Elsevier ButterworthHeinemann; 2007
[4] Rowe F. Clinical Orthoptics. 2nd ed. Oxford:
Blackwell Publishing Ltd; 2004
[5] Ludvigh E. Amount of eye movement objectively
perceptible to the unaided eye. Am J Ophthalmol.
1949 32:649
Acknowledgements
The authors are grateful to the Oxford Orthoptic
Department for assistance with figures 3, 4 and 5
and to Ms Alexa Wilder and the Horton Orthoptic
Department for help with figure 6. We would also
like to thank Ms Jo Hunter for her help with literature
research.
Conflicts of interest
None declared
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1
2
3
4
5*
6*
7
8
9
History, especially of double vision - horizontal, vertical and tilting effect on gaze
Determine visual acuity (VA) in each eye
Measure refractive error
Inspect eyes and head position
Determine presence of ocular misalignment
Measure misalignment, direction and degree
Ocular motility
Suppression and level of stereopsis (depth perception)
Pupil dilation and fundoscopy to detect cause of reduced vision such as
retinal scar, retinoblastoma , and hence ocular misalignment
* Where the cover test is usually performed
Table 1 Full assessment of suspected ocular misalignment. Modified based on [1].
A penlight is shone at the patient from about an arm’s length away and equidistance to the patient’s eyes. In
manifest squints, the corneal reflections in both eyes will be asymmetrical - light reflection will be sited offcentre in the deviating eye.
Hirschberg test: Hirschberg found that each mm of cornea light reflection displacement from the centre is
equivalent to 7 degrees of ocular deviation from the visual axis. Further, estimations of squint amplitude can be
derived from the position of the light reflection in relation to the pupil, iris and limbus (du Bois-Reymond) [1]
(Figure 7).
Krimsky test: Prisms are placed in front of the fixating eye until corneal light reflections in both eyes are
symmetrical. This gives a more precise measurement of the squints than the Hirschberg test.
Table 2 Corneal reflection test.
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